Apparatus including spark gap electrodes for spark testing plastic layers



May 3, 1966 A. o. MACK ETAL 3,249,862 APPARATUS INCLUDING SPARK GAP ELECTRODES FOR SPARK TESTING PLASTIC LAYERS Filed June 30. 1960 y K a A u 20 I 46 L Z \V\\ Z A 70 Probe Power 54 INVENTORS,

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United States Patent APPARATUS INCLUDING SPARK GAP ELEC- TRODES FOR SPARK TESTING PLASTIC LAYERS Arnold O. Mack and Harold L. Hearns, Midland, M1ch., assignors to The Dow Chemical Company, Midland, Mich., a corporation of Delaware Filed June 30, 1960, Ser. No. 39,933 4 Claims. (Cl. 324-54) This invention relates to sparking apparatus and particularly to apparatus for spark testing to detect small holes in layers of plastic.

It is customary to use high frequency spark testing' devices to detect small holes in plastic linings of tanks or other storage vessels. Such testing is done by drawing an electrode energized by high frequency high voltage over the surface and observing any arcs between the movable electrode and the metal to which the plastic liner is adhered. Any arcing, of course, indicates a perforation in the liner. -A very substantial shortcoming to such testing has been the problem .of burning holes through a previously unperforated part ofthe liner if the electrode were not rapidly moved across the surface of the liner. That is, a hole often is bumed'through the liner if the electrode is momentarily held in one position along the liner.

Accordingly, a principal object of this invention is t6 provide an improved apparatus for use in spark testing plastic liners for apertures. 7

Another object of this invention is to provide improved electrode apparatus in which spark intensity is easily controlled.

In accordance with this invention, there is provided sp'ark testing apparatus in which a high frequency sparking transformer having an output voltage control is secured mechanically to but insulated electrically from a probing bar which is rod-like in form and several inches in length. Spark gap electrodes, one secured to the transformer output part and one secured to the probing bar, provide breakdown path means by which the probing bar is energized by the transformer part.

The invention, as well as additional objects and advan tages thereof, will best be understood when the following detailed description is read in connection with the accompanying drawing, in which:

FIG. 1 is a side elevational view, in section, of apparatus made in accordance with this invention, and

FIG. 2 is a sectional view taken along the line 2-2 of FIG. 1, and FIG. 3 is a schematic diagram of a transformer high voltage device suitable for use with this invention.

Referring to the drawing, there is shown spark testing apparatus, indicated generally by the numeral 10, comprising a high frequency transformer unit 12 which is encased in the handle of the apparatus 10. A voltage adjusting knob 14 is coupled to the rear end of the transformer 12, the electrical energizing cord 16 extending through the knob 14. FIG. 3 shows an auto transformer 50 whose input 52 is coupled to a power source (not shown) and whose output lead 56 is to be coupled to the output electrode 20. The input voltage to the transformer is varied by means of rheostat 54 controlled by knob 14.

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At the forward end 18 of the transformer a rod-like output electrode 20 protrudes and has a transversely disposed metal cross bar 22 mechanically and electrically coupled thereto, the coupling to the cross bar being made at or near the central section of the cross bar.

A pair of insulating standoff elements 24, 26 are mechanically coupled to the crossbar 22 by means of bolts 28, 30. p

A probe bar 32, made of metal and presenting a straight line surface 34 over a substantial part of its length, is secured mechanically to the insulating standoff elements 24, 26 by means of countersunk bolts 36, 38.

The surface 34 is adapted to slide over the surface of an insulating liner 40 (Saran rubber, Heresite, polyethylene film, for example) which is contiguous to a metal base element 42.

At one end of the probe bar is disposed an upwardly extending discharge electrode 44 which has a somewhat pointed end. The adjacent end of the cross bar 22 has a similar downwardly extending somewhat pointed electrode tip 46. The two electrodes 44, 46 constitute a spark gap arrangement with fixed spacing, usually of the order of one-half inch.

In operation the transformer unit is energized, the output voltage being adjusted to approximately the minimum voltage which will maintain continuous sparking across the gap electrodes 44, 46. The tool is then drawn across the surface of an insulating layer 40. When the probe bar approaches a small pin hole in the layer 40, at least one, and usually a plurality of thin but visible sparks extends from the bar 32 to the imperfection (not shown) in the layer 40, giving positive identification as to the location of the imperfection.

However, unlike conventional spark probing electrodes, the spark is strong enough to give obvious indication of the presence in the liner 40 of imperfection, yet will not burn its way through the surface even though the probe electrode 32 be left in the same position on the layer 40 for several seconds (up to a minute with most films).

Thus, the spark testing apparatus of this invention obviates the primary disadvantage of prior art sparking probes in that it does not cause the formation of additional imperfections in the layer 40.

In using the prior art probes inadvertent stopping of movement of the probe sometimes resulted in the creation of as many new imperfections as there were old imperfections to be detected.

It should be noted that the spark gap electrode elements 44, 46 may be disposed at other locations along the cross bar 22 of probe bar 34, the end location being merely the easiest in which tolocate them.

What is claimed is:

1. Spark testing apparatus for detecting small perforations in insulated layers disposed contiguous to an electrical conductor, comprising a high frequency transformer having an output electrode, said transformer having voltage adjusting means coupled thereto, a metal cross bar member, said cross bar member being mechanieally and electrically coupled to said output electrode, a pair of standoff insulators, one of said standoff insulators being secured to said cross bar adjacent to each end thereof, said probe bar being secured to said standoff insulators, and a pair of spark gap electrodes, one of said having an insulation contacting surface along the length References Cited by the Examiner theriof, andl atpacilr olfj'spa-rk gap deliectro iles, onebcgf saig' UNITED STATES PATENTS spar gap e co r0 es eing secure 0 sa1 cross r an the other of said spark gap electrodes being secured to fi g 2 2 garvoe 324-54 said probe bar, the spacing between said spark gap elec- 5 29 19 g roger 54 trodes being the minimum air gap spacing between the 3/194 Groven 324-54 cross bar and the probe ban 2,750,562 6/1956 Staff 32454 2. Apparatus in accordance with claim 1, wherein said FOREIGN PATENTS spark gap electrodes are an integral part of the cross bar 760,040 10/1953 Germany.

and the probe bar. 10

3. Apparatus in accordance with claim 1, wherein said WALTER L CARLSON, P i E i gig; bar is straight along a substantial part of 1ts RALPH G. NILSON Examiner- 4. Apparatus in accordance with claim 1, wherein C- R- CAMPBELL, G. R. STRECKER, probe bar is made of metal. 15 Assistant Examiners- 

1. SPARK TESTING APPARATUS FOR DETECTING SMALL PERFORATIONS IN INSULATED LAYERS DISPOSED CONTIGUOUS TO AN ELECTRICAL CONDUCTOR, COMPRISING A HIGH FREQUENCY TRANSFORMER HAVING AN OUTPUT ELECTRODE, SAID TRANSFORMER HAVING VOLTAGE ADJUSTING MEANS COUPLED THERETO, A METAL CROSS BAR MEMBER, SAID CROSS BAR MEMBER BEING MECHANICALLY AND ELECTRICALLY COUPLED TO SAID OUTPUT ELECTRODE, A PAIR OF STANDOFF INSULATORS, ONE OF SAID STANDOFF INSULATORS BEING SECURED TO SAID CROSS BAR ADJACENT TO EACH END THEREOF, SAID PROBE BAR BEING SECURED TO SAID STANDOFF INSULATORS, AND A PAIR OF SPARK GAP ELECTRODES, ONE OF SAID HAVING AN INSULATION CONTACTING SURFACE ALONG THE LENGTH THEREOF, AND A PAIR OF SPARK GAP ELECTRODES, ONE OF SAID SPARK GAP ELECTRODES BEING SECURED TO SAID CROSS BAR AND THE OTHER OF SAID SPARK GAP ELECTRODES BEING SECURED TO SAID PROBE BAR, THE SPACING BETWEEN SAID SPARK GAP ELECTRODES BEING THE MINIMUM AIR GAP SPACING BETWEEN THE CROSS BAR AND THE PROBE BAR. 